Armstrong (pectoral) was placed on injured reserved Tuesday, Carl Steward of the Bay Area News Group reports.
Armstrong suffered the injury in a Week 2 loss to the Panthers and will now potentially spend the rest of the season on the sidelines. The specifics of the injury have not been announced, meaning Armstrong could be considered a candidate for the new IR return rule. However, if the injury is a torn pectoral, it seems unlikely the linebacker would be able to heal in time for the latter part of the season. The 49ers have not announced any corresponding roster moves to fill Armstrong's spot on the roster. Before suffering the injury, the 27-year-old recorded eight tackles, two pass deflections, and one interception in less than two games, leaving a sizable hole in San Francisco's defense. Gerald Hodges is the most likely candidate to take over inside linebacker duties in Armstrong's absence.